Assay of a protein or the like in a sample is primarily performed by immunoassay these days. As immunoassay, a variety of methods are known and have been put to practical use. Whichever method is chosen, a specific antibody against a target substance is used. Creation of a specific antibody against a target substance can be conducted by a method known per se in the art, but is laborious so that the specific antibody is costly.
On the other hand, there are also known aptamers which are nucleic acid molecules that specifically bind to desired molecules. An aptamer that specifically binds to a desired target molecule can be created by a method called “SELEX (Systematic Evolution of Ligands by EXponential Enrichment)” (Non-patent Document 1). According to this method, the target molecule is immobilized on a carrier, to which a nucleic acid library including a huge variety of nucleic acids having random base sequences is added. One of the nucleic acids, which has bound to the target molecule, is collected, amplified by PCR, and is again added to the carrier with the target molecule immobilized thereon. By repeating this step 10 times or so, the aptamer having high binding ability to the target molecule is concentrated. Its base sequence is then determined to acquire the aptamer that recognizes the target molecule. It is to be noted that the above-described nucleic acid library can be readily prepared by binding nucleotides at random in an automated chemical synthesizer for nucleic acids. By the method that makes use of a nucleic acid library having random base sequences and makes positive use of accidents as described above, it is possible to create an aptamer that specifically binds to a desired target substance. Further, an aptamer generally has a single-stranded region. It is also known to create, by a modified SELEX method, an aptamer called “structure-switching aptamer” that an oligonucleotide complementary to the single-stranded region is hybridizable with the single-stranded region when the aptamer is in a state that it is not bound to a target substance but is not hybridizable with the single-stranded region when the aptamer is in a state that it is bound to the target substance (Non-patent Document 2).    Patent Document 1: Japanese Patent Laid-open No. 2003-294679    Patent Document 2: Japanese Patent Laid-open No. 2003-294680    Patent Document 3: Japanese Patent Laid-open No. 2003-294681    Non-patent Document 1: Tuerk, C. and Gold, L., Science, 294, 505-510 (1990)    Non-patent Document 2: Angew. Chem. Int. Ed., 44, 1061-1065 (2005)    Non-patent Document 3: Kazunori Ikebukuro, et al., Nucleic Acids Research, 33(12), e108    Non-patent Document 4: Wiegand T. W., Williams P. B., Dreskin S. C., Jouvin M. H., Kinet J. P., Tasset D., High-affinity oligonucleotide ligands to human IgE inhibit binding to Fc epsilon receptor I, JOURNAL OF IMMUNOLOGY, 157(1), 221-230 (JUL 1, 1996)